1. Field of the Invention
The present invention relates generally to a device and method for DC offset cancellation, and more particularly to a DC offset cancellation device and method used in direct down conversion receivers.
2. Description of Related Art
Direct down conversion receivers are generally applied in wireless communications for preamble processing such as receiving a wireless radio frequency signal, converting, amplifying and filtering the wireless radio frequency signal so as to obtain corresponding digital baseband signal which is further provided to a next stage circuit.
However, during the conversion of the analog signal to the digital signal, DC offset often occurs due to the following reasons: (1) transistors in the signal path do not entirely match; (2) signal leakage occurs to the local oscillators and thus produces DC offset; and (3) reflected signal is received again by the antenna and produces DC offset.
When the signal in a direct down conversion receiver has DC offset, digital baseband signal produced in the direct down conversion receiver is distorted. The amplifiers or filters in the direct down conversion receivers may be saturated due to the DC offset, thus adversely affecting normal operation of the direct down conversion receiver. Therefore, it is important to provide a DC offset correction mechanism or DC offset cancellation mechanism for wireless receivers.
FIG. 1a shows a basic structure of a direct down conversion receiver. The direct down conversion receiver 10 comprises: a mixer 11, a gain amplifier 12, a filter 13 and an analog-to-digital converter 14. During operation, the mixer 11 receives a radio frequency signal and a local oscillation signal to produce a baseband signal, and then the baseband signal is amplified and filtered, and converted to digital baseband signal through the analog-to-digital converter 14.
FIGS. 1b and 1c show a first DC offset correction structure and a second DC offset correction structure in the prior art, respectively.
As shown in FIG. 1b, a variable high pass module 15 is connected between the amplifier 12 and the filter 13 in series to cancel DC offset in the direct down conversion receiver.
Alternatively, as shown in FIG. 1c, a high pass feedback loop having similar effect is provided to cancel DC offset in the direct down conversion receiver.
However, the above-described two techniques have long reaction time; in other words, it requires long reaction time to completely cancel DC offset, which accordingly cannot meet the demand for high communication efficiency of current communication devices.
Therefore, it is in urgent need to provide a DC offset correction technique with short reaction time and high correction efficiency.